The only surviving example of a legendary stamp that sold for one cent in 1856 has gone under the hammer for $9.5m (£5.6m), setting a new record for the most expensive stamp sold at auction and the most valuable object by weight and size.

Sotheby's sale of the British Guiana one-cent magenta postage stamp reinforces its reputation as the world's most famous and valuable.

Recently owned by an American millionaire who died four years ago in a prison cell, the stamp was one of an emergency printing of several denominations by the local Official Gazette newspaper in British Guiana in 1856, when storms delayed a shipment from the UK and the postmaster was in danger of entirely running out of stamps.

The stamp was sold by the estate of John du Pont, an heir to the DuPont chemical fortune, who paid nearly $1m for it in New York in 1980. He won an award in 1986 for a display of British Guiana stamps at an international exhibition in 1986, the last time the stamp was seen in public before the current promotional tour by the auction house.

Ten years later, suffering from severe mental illness, Du Pont shot dead the Olympic gold-medal wrestler Dave Schultz – after many years of sponsoring and helping in the training of Schultz and other US Olympic athletes. Du Pont is portrayed by Steve Carrell in a film, Foxcatcher, which is set to be released this year in the US.

Some of the money raised from the sale is going to another of the causes Du Pont championed, the Eurasian Pacific Wildlife Conservation Foundation.

The stamp's first owner was Vernon Vaughan, a 12-year-old Scottish schoolboy living with his family in British Guiana who found it among his uncle's papers in 1873. In the 1880s, it was bought by Count Philippe la Renotière von Ferrary, one of the wealthiest men in Europe.

David Redden, Sotheby's chairman and a books specialist, said: "I have been with Sotheby's all my working life, but before I knew about the world's greatest works of art, before I knew about the Mona Lisa or Chartres Cathedral, I knew about the British Guiana."